Coating compositions, such as cationic electrodepositable compositions, sometimes include colorant and/or filler particles to impart color and/or performance properties in the resulting coating. Pigment particles tend to have a strong affinity for each other and, unless separated, tend to clump together to form agglomerates. Therefore, these agglomerates are often dispersed in a resinous grind vehicle and, optionally, dispersants by milling or grinding using high shear techniques to break up the agglomerates. If nano-sized pigment particles are desired, further milling is often required to obtain the desired particle size.
Pigments and fillers usually consist of solid crystalline particles ranging in diameter from about 0.02 to 2 microns (i.e., 20 to 2000 nanometers). Agglomeration is a serious problem for nano-sized particle pigments and filler materials (such as carbon black) in particular because these nanoparticles have a relatively large surface area. Thus, acceptable dispersion of such nanoparticles often requires an inordinate amount of resinous grind vehicle and/or dispersant to effect de-agglomeration and to prevent subsequent re-agglomeration of the nanoparticles.
The presence of such high levels of resinous grind vehicles and dispersants, however, in the final coating composition can be detrimental to the resultant coating. For example, high levels of dispersants have been known to contribute to water sensitivity of the resultant coating. Also, some resinous grind vehicles, for example, acrylic grind vehicles, can negatively impact coating performance properties such as chip resistance and flexibility.
Electrodepositable coating compositions are often used to provide coatings for protection of metal substrates, such as those used in the automobile industry. Electrodeposition processes often provide higher paint utilization, outstanding corrosion protection, low environmental contamination, and/or a highly automated process relative to non-electrophoretic coating methods.
In the electrodeposition process, an article having an electroconductive substrate, such as an automobile body or body part, is immersed into a bath of a coating composition of an aqueous emulsion of film forming polymer, the electroconductive substrate serving as a charge electrode in an electrical circuit comprising the electrode and an oppositely charged counter-electrode. An electrical current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a coating having the desired thickness is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
It would also be desirable to provide an aqueous dispersion of resin-enclosed particles, wherein re-agglomeration of the particles is minimized, and which is suitable for use in preparing cationic electrodepositable coating compositions that exhibit the advantages of electrodepositable coating compositions. It would also be desirable to provide such cationic electrodepositable coating compositions that are capable of producing color-imparting non-hiding coating layers.